1. Field of the Invention
The present invention relates generally to reflective optical filters, and particularly to an asymmetric low dispersion Bragg grating filter.
2. Technical Background
Wavelength division multiplexing (WDM) is pervasive in optical communication systems. Key optical components in these optical system are those that perform the functions of combining different wavelength channels and splitting wavelength channels.
Bragg grating structures are useful as spectral filters for WDM and other optical signal processing applications. In these types of applications, the Bragg grating filters should have low insertion loss, a flat-top spectral response, steep slopes to minimize crosstalk and a linear phase response to avoid transmission penalties due to signal degradation arising from filter dispersion. Although a reflection filter based on Bragg gratings can be designed to have a near-ideal spectral response, the corresponding phase response becomes increasingly nonlinear (i.e., increased dispersion) as the band edges are approached, thereby reducing the useful filter bandwidth. While the amplitude response of such optical filters has received much attention, the phase response has only recently been investigated in the context of optical communication systems.
Reflective filters based on conventional Bragg gratings have dispersion characteristics which are determined by the transmission spectrum of the grating through a relation which is dictated by causality of such filters. As a consequence of this, a band-pass filter with sharp falling edge in spectrum will suffer a larger in-band dispersion. The in-band dispersion is typically worst towards the edge of the band. In this case, the filter exhibits an effective dispersion-limited filter bandwidth less than the bandwidth of a conventional amplitude-limited filter. However, for a fixed amplitude response in a Bragg grating filter, dispersion of the filter can be improved over the causality-defined dispersion limit. Since reflective filters generally use only reflection from one end of the Bragg grating, dispersion of the reflection can be decreased at one end of the grating at the expense of increased dispersion at the other end of the grating. This unique feature of the Bragg grating is very profound, since it opens a way to make a dispersionless ideal filter which can play a very important role in the field of optical communications.